In a solar cell module used in photovoltaic power generation, it is general to use an EVA sheet on both sides of the cell for cell protection and additionally a transparent glass substrate on one side facing sunlight and a sheet having excellent moisture-proof and weather resistant properties on the other side, respectively are laminated. The lamination is carried out by layering a transparent substrate, an EVA sheet, a cell, an EVA sheet and a gas-proof sheet, and then crosslinking and binding them together by heat at a certain temperature and pressure.
Since high transparency, adhesiveness and weather resistance after cross-linking is generally required in the EVA sheet for a solar cell encapsulant, various additives such as a crosslinking agent, a co-crosslinking agent, a silane coupling agent, an antioxidant, a light stabilizer, a UV absorber and the like are used, wherein the additives are dry-blended with EVA by a Henschel mixer or a tumbler, or fed to EVA by a separate feeding device for additives, and then melt-mixed with the EVA at a temperature that is the melting point of EVA or higher but not more than the degradation temperature of the crosslinking agent, organic peroxide so as to form an EVA sheet for a solar cell encapsulant.
Since organic peroxides used in a solar cell encapsulant are generally those having a low degradation temperature (1 hour half-life temperature) such as no more than 150° C. for increasing productivity during lamination and cross-linking and reducing the degree of yellowing and the amount of residual peroxides after crosslinking, and since the temperature at which the degradation starts is very low in practice, there is limitation in increasing the process temperature and thus the sheet manufacture is generally carried out by melt-mixing at 120° C. or less. In the meanwhile, if the melt-mixing process is carried out under the conditions for raising shear force in order to increase productivity, heat is generated which may promote the degradation of the organic peroxides, thereby leading pre-crosslinking disadvantageously. Therefore, there have been limitations in making an improvement in productivity.
In the meantime, according to conventional methods for manufacturing an EVA sheet for a solar cell encapsulant wherein all the components to be blended including an EVA resin, additives, a crosslinking agent and the like are mixed and melt-mixed together at once under the conditions involving low shear force, some additives such as an antioxidant or a light stabilizer are not able to be uniformly dispersed in the EVA phase since they have higher melting point that is near the EVA sheet manufacturing temperature or more and thus hardly melt.
Further, when an EVA resin which has a relatively low vinyl acetate content and melting point is used under the conditions involving low shear force and not more than the temperature of the organic peroxide degradation temperature, the dispersion of additives becomes bigger problem, and a problem such as the generation of unmelted gels owing to disuniform melting of the EVA resin used further occurs. However, if the process temperature is raised to solve the problem, the antioxidant and the light stabilizer and the like may directly contact-react with the organic peroxides which can cause reduction of the antioxidation effect, and the amount of organic peroxide for achieving the desired level of crosslinking is further required for complementing the loss of organic peroxide, which may result in yellowing after crosslinking or generation of bubbles. In other case, when additives are fed through a separate feeder to an extruder for the extrusion of an EVA sheet, in particular in case of powder type additives, not liquid type, further problem of sticking of such powder type additives onto the wall of the feeder owing to its intrinsic powder properties or bridging phenomenon may occur, which can prevent feeding of the additives in precise amount.